Many of DC power supply apparatuses, such as ones for a welder, a cutter, a plating system and a communications system, a battery charger and a DC power supply for use in igniting an arc lamp, include a DC-to-high-frequency converter so that the DC power supply apparatus with such converter can be made small in size and light-weighted.
A DC power supply apparatus of this type may include an input-side AC-to-DC converter which converts an input commercial AC voltage to a DC voltage. The DC voltage is, then, converted into a high-frequency voltage of, for example, from several kilohertz to 100 KHz, by a DC-to-high-frequency converter. The high-frequency voltage is, then, applied to a primary winding of a high-frequency transformer. This causes a high-frequency voltage of a predetermined magnitude to be developed in a secondary winding of the transformer. The high-frequency voltage developed in the secondary winding is converted back into a DC voltage by a high-frequency-to-DC converter. The resultant DC voltage is applied to a load.
The DC-to-high-frequency converter includes at least one of semiconductor switching devices, such as IGBTs, FETs and bipolar transistors. The semiconductor switching device is PWM controlled, i.e. rendered conductive when a conduction control signal is applied thereto, and is rendered nonconductive when the conduction control signal is removed, to thereby convert a DC voltage into a high-frequency voltage.
Load current supplied to the load is detected by a current detector, and a load-current representative signal is applied from the current detector to a controller. The controller supplies the semiconductor switching device in the DC-high-frequency converter with the conduction control signal at intervals for PWM controlling the semiconductor switching device in such a manner that the load-current representative signal becomes equal to a predetermined reference signal.
The magnitude of a commercial AC voltage differs from state to state and area to area. For example, a commercial AC voltage in one state may be 180 V, which, in other state, it may be 200 V or 220 V. Depending on the magnitude of the input commercial AC voltage applied, the magnitude of the DC output voltage of the input-side AC-to-DC converter, the peak value of the high-frequency voltage from the DC-to-high-frequency converter, the magnitude of the voltage across the secondary winding of the transformer and the magnitude of the DC output voltage of the high-frequency-to-DC converter vary.
For example, a solid line, a dot-and-dash line and a dotted line shown in FIG. 8 represent static output voltage-output current characteristics exhibited by a DC power supply apparatus of the above-described type when a commercial AC voltage of 180 V, 200 V and 220 V is applied to it, respectively. As is understood, as the commercial AC voltage is higher, the output voltage becomes larger.
Due to the alternation of the semiconductor switching device between conduction and non-conduction, voltage pulses are repetitively applied to the high-frequency transformer as shown in FIG. 9. There must be a vacant period between the respective voltage pulses. The shortest possible length td of the vacant period remains the same for a low AC supply voltage of, e.g. 180 V, represented by a solid line, an intermediate-magnitude AC supply voltage of e.g. 200 V represented by a dot-and-dash line, and a high AC supply voltage of e.g. 220 V as represented by a dotted line.
The number, n, of turns of the winding of the high-frequency transformer, which involves no magnetic saturation, is determined by the following expression (1). ##EQU1##
In this expression (1), E.sub.max is the highest possible voltage applied to the transformer, Ton.sub.max is the longest time period during which the input voltage is applied to the transformer, .DELTA.B is the magnetic flux density in the core of the transformer, and S is the cross-sectional area of the core. Usually, the time period td is constant, and the period of the conduction control signal is constant. Accordingly, the time period Ton.sub.max is constant. Therefore, if the DC power supply apparatus is used selectively with commercial AC voltages of different values, the highest possible voltage E.sub.max, which is determined by the highest one of the commercial AC voltages, becomes higher. As the highest possible voltage E.sub.max is higher, the number, n, of turns and the cross-sectional area of the core increase, which results in increase of the size of the transformer.
Thus, even if the DC power supply apparatus employs a DC-to-high-frequency converter for converting a DC voltage to a high-frequency voltage as well as a high-frequency transformer for the purpose of reducing the size of the apparatus, it cannot be downsized if it is a DC power supply apparatus which is to be used with commercial AC voltages of different values.
For the purpose of downsizing, some power supply apparatuses employ a voltage-boosting or voltage-lowering converter disposed between the output of the input-side AC-to-DC converter and the input of the DC-to-high-frequency converter, which is so controlled as to develop a constant output voltage. In this case, since the voltage-boosting or voltage-lowering converter must control relatively large current, it must employ a relatively large switching control device. Also, this approach entails the use of complicated circuitry.
An object of the present invention is to provide an arrangement of a DC power supply apparatus, which does not necessitate the use of complicated circuitry and which can provide a small-sized DC power supply apparatus.